Method and apparatus for determining oversensing in a medical device

ABSTRACT

A method and apparatus for determining oversensing in a medical device that includes sensing cardiac signals and detecting cardiac events via a first electrode configuration, determining the presence of an episode event in response to the detected cardiac events, and sensing the cardiac signals via a second electrode configuration. Amplitudes associated with a predetermined number of cardiac signals sensed via the second electrode configuration corresponding to the event episode are determined, and delivery of therapy is controlled in response to the determined amplitudes.

RELATED APPLICATION

The present invention claims priority and other benefits from U.S.Provisional Patent Application Ser. No. 60/632,000, filed Dec. 1, 2004,entitled “IDENTIFICATION OF OVERSENSNING IN A MEDICAL DEVICE”,incorporated herein by reference in its entirety. Cross-reference ishereby made to commonly assigned related U.S. Pat. No. 7,167,747,incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention generally relates to medical devices, and moreparticularly relates to reducing the effects of oversensing in medicaldevices.

BACKGROUND OF THE INVENTION

Implantable medical devices (IMDs) have many functions including thedelivery of therapies to cardiac patients, neuro-stimulators, muscularstimulators, and others. Application of the present invention isdescribed below in the context of implantable cardiac pacemakers and/ordefibrillators, it being understood that the principles herein may haveapplicability to other implantable medical devices as well.

An example of an implantable medical device (IMD) includes a devicecommonly referred to as a pacemaker, which is used to stimulate theheart into a contraction if the sinus node of the heart is not properlytiming, or pacing, the contractions of the heart. Modern implantablemedical devices also perform many other functions beyond that of pacing.For example, a pacemaker/cardioverter/defibrillator (PCD) performstherapies such as defibrillation and cardioversion as well as providingseveral different pacing therapies, depending upon the needs of the userand the physiologic condition of the user's heart.

In typical use, a PCD is implanted in a convenient location usuallyunder the skin of the user and in the vicinity of the one or more majorarteries or veins. One or more electrical leads connected to the PCD areinserted into or on the heart of the user, usually through a convenientvein or artery. The ends of the leads are placed in contact with thewalls or surface of one or more chambers of the heart, depending uponthe particular therapies deemed appropriate for the user.

One or more of the leads is adapted to carry a current from the PCD tothe heart tissue to stimulate the heart in one of several ways, againdepending upon the particular therapy being delivered. The leads aresimultaneously used for sensing the physiologic signals provided by theheart to determine when to deliver a therapeutic pulse to the heart, andthe nature of the pulse, e.g., a pacing pulse or a defibrillation shock.

In the sensing mode, sense amplifiers coupled to the leads provideamplification to electrogram signals picked up by the sensing electrodesin the heart. The analysis of these signals by the PCD determineswhether a therapy (a pacing pulse or a defibrillator shock) should beadministered. If erroneous signals are detected by the PCD, anunnecessary therapy may be administered, providing an unnecessary pacingpulse or defibrillator shock to the patient. One cause of erroneousinterpretation of sensing signals is oversensing, that is, the falsedetection of a depolarization event.

Sensing can be accomplished in a number of ways. If two bipolar leadsare used, one lead is typically placed within the right ventricle of theheart and a second lead is placed within the right atrium of the heart.Both leads include two sensing elements: a tip electrode that isattached to the wall or surface of the heart, and a ring electrode thatis located on the lead but removed some distance from the tip electrode.A high voltage coil located on one or both of the leads can also be usedfor sensing, as can the implanted PCD can itself. Some cardiacconditions require sensing at both the right ventricle and the rightatrium, and still others may add sensing at the left ventricle via athird lead positioned within the coronary sinus. As a result, there area large number of sensing paths and combinations available for use,depending upon the configuration and programming of the specificimplantable device, and some PCDs can be configured to switch to analternate sensing path if the primary path is determined to be faulty.

Oversensing of cardiac waves may be caused by lead fractures (e.g.,conductor break, insulation break, adaptor failure), connectors (e.g.loose set screw), T-wave oversensing, R-wave oversensing,electromagnetic interference (EMI), and myopotentials. In the past,oversensing problems (e.g., myopotentials, T-wave oversensing) have beendealt with by modifying sense amplifiers, filters and PCD leadelectrodes. In currently used PCDs the sense amplifiers have selfadjusting sensing thresholds for sensitivity, so oversensing oftenoccurs as a result of lead failure. Upon detection of an R-wave, thethreshold of the sense amplifier is raised to about 75% of the R-wave.The sense amplifier threshold then decays until the next R-wave issensed. In this manner the sensing threshold of the sense amplified iscontinually adjusted to allow for variations in the sensed strength ofthe R-wave. However, there is still room for improved techniques foreliminating the effects of oversensing.

Accordingly, it is desirable to provide an additional mechanism fordealing with oversensing of depolarization events. In addition, it isdesirable to provide an algorithm to be incorporated into detectionalgorithms in the IMD to prevent the detection and erroneous applicationof therapies for detected episodes caused by oversensing. Furthermore,other desirable features and characteristics of the present inventionwill become apparent from the subsequent detailed description and theappended claims, taken in conjunction with the accompanying drawings andthe foregoing technical field and background.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present invention will be readily appreciated as theybecome better understood by reference to the following detaileddescription when considered in connection with the accompanyingdrawings, wherein:

FIG. 1 is a schematic diagram of a pacemaker/cardioverter/defibrillatorand lead set of a type in which the present invention may usefully bepracticed;

FIG. 2 is a functional schematic diagram of an implantablepacemaker/cardioverter/defibrillator of the type illustrated in FIG. 1,in which the present invention may usefully be practiced;

FIG. 3 is a portion of an electrogram showing near-field and far-fieldR-wave sensing pulses where there is an indication of a false positivenear-field pulse;

FIG. 4 is a portion of an electrogram showing near-field and far-fieldR-wave sensing pulses where there is an actual cardiac episode requiringtherapy;

FIG. 5 is a flow chart of a method of delivering a therapy in animplantable medical device, according to an embodiment of the presentinvention;

FIGS. 6A and 6B are graphical representations of a determination of abaseline measure of a far-field signal according to an embodiment of thepresent invention; and

FIG. 7 is a flow chart of a method of delivering a therapy in a medicaldevice, according to an embodiment of the present invention.

FIG. 8 is a graphic representation of maximum and minimum amplitudes ofsensed events for delivery of therapy by a medical device according tothe present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic diagram of a pacemaker/cardioverter/defibrillatorand lead set of a type in which the present invention may usefully bepracticed. The ventricular lead includes an elongated insulative leadbody 16, carrying three mutually insulated conductors. Located adjacentthe distal end of the lead are a ring electrode 24, an extendable helixelectrode 26, mounted retractably within an insulative electrode head28, and an elongated coil electrode 20. Each of the electrodes iscoupled to one of the conductors within the lead body 16. Electrodes 24and 26 are employed for cardiac pacing and for sensing ventriculardepolarizations. At the proximal end of the lead is a bifurcatedconnector 14, which carries three electrical connectors, each coupled toone of the coiled conductors.

The atrial/SVC lead includes an elongated insulative lead body 15, alsocarrying three mutually insulated conductors. Located adjacent theJ-shaped distal end of the lead are a ring electrode 21 and anextendible helix electrode 17, mounted retractably within an insulativeelectrode head 19. Each of the electrodes is coupled to one of theconductors within the lead body 15. Electrodes 17 and 21 are employedfor atrial pacing and for sensing atrial depolarizations. An elongatedcoil electrode 23 is provided, proximal to electrode 21 and coupled tothe third conductor within the lead body 15. At the proximal end of thelead is a bifurcated connector 13, which carries three electricalconnectors, each coupled to one of the coiled conductors.

The coronary sinus lead includes an elongated insulative lead body 6,carrying one conductor, coupled to an elongated coiled defibrillationelectrode 8. Electrode 8, illustrated in broken outline, is locatedwithin the coronary sinus and great vein of the heart. At the proximalend of the lead is a connector plug 4 that carries an electricalconnector, coupled to the coiled conductor.

The pacemaker/cardioverter/defibrillator 10 includes a hermeticenclosure or housing 11 containing the electronic circuitry used forgenerating cardiac pacing pulses for delivering cardioversion anddefibrillation shocks and for monitoring the patient's heart rhythm.Pacemaker/cardioverter/defibrillator 10 is shown with the lead connectorassemblies 4, 13 and 14 inserted into the connector block 12, whichserves as a receptacle and electrical connector for receiving theconnectors 4, 13 and 14 and interconnecting the leads to the circuitrywithin housing 11. An optional sensor 30 is illustrated schematically bybroken outline, and may include one or more of an activity sensor,respiration sensor (potentially from impedance), accelerometer-basedposture detector, heart rate detector, ischemia detector and otheravailable physiological sensor known in the art for measuring hearthemodynamics and may be a piezoelectric transducer as known in the art.Sensor 30 may be used, for example, to regulate the underlying pacingrate of the device in rate responsive pacing modes.

Optionally, insulation of the outward facing portion of the housing 11of the pacemaker/cardioverter/defibrillator 10 may be provided or theoutward facing portion may instead be left uninsulated, or some otherdivision between insulated and uninsulated portions may be employed. Theuninsulated portion of the housing 11 optionally serves as asubcutaneous defibrillation electrode, used to defibrillate either theatria or ventricles. Other lead configurations and electrode locationsmay of course be substituted for the lead set illustrated. For example,atrial defibrillation and sensing electrodes might be added to eitherthe coronary sinus lead or the right ventricular lead instead of beinglocated on a separate atrial lead, allowing for a two lead system.

FIG. 2 is a functional schematic diagram of an implantablepacemaker/cardioverter/defibrillator of the type illustrated in FIG. 1,in which the present invention may usefully be practiced. This diagramshould be taken as exemplary of one type of anti-tachyarrhythmia devicein which the invention may be embodied, and not as limiting, as it isbelieved that the invention may usefully be practiced in a wide varietyof device implementations, including devices providing therapies fortreating atrial arrhythmias instead of or in addition to ventriculararrhythmias, cardioverters and defibrillators which do not provideanti-tachycardia pacing therapies, anti-tachycardia pacers which do notprovide cardioversion or defibrillation, and devices which deliverdifferent forms of anti-arrhythmia therapies such nerve stimulation ordrug administration.

The device is provided with a lead system including electrodes, whichmay be as illustrated in FIG. 1. Alternate lead systems may of course besubstituted. If the electrode configuration of FIG. 1 is employed, thecorrespondence to the illustrated electrodes is as follows. Electrode311 corresponds to an electrode formed along the uninsulated portion ofthe housing of the implantable pacemaker/cardioverter/defibrillator.Electrode 320 corresponds to electrode 20 and is a defibrillationelectrode located in the right ventricle. Electrode 310 corresponds toelectrode 8 and is a defibrillation electrode located in the coronarysinus. Electrode 318 corresponds to electrode 28 and is a defibrillationelectrode located in the superior vena cava. Electrodes 324 and 326correspond to electrodes 24 and 26, and are used for sensing and pacingin the ventricle. Electrodes 317 and 321 correspond to electrodes 19 and21 and are used for pacing and sensing in the atrium.

Electrodes 310, 311, 318 and 320 are coupled to high voltage outputcircuit 234. Electrodes 324 and 326 are coupled to the R-wave amplifier200, which preferably takes the form of an automatic gain controlledamplifier providing an adjustable sensing threshold as a function of themeasured R-wave amplitude. A v-sense signal is generated on R-out line202 whenever the signal sensed between electrodes 324 and 326 exceedsthe present sensing threshold.

Electrodes 317 and 321 are coupled to the P-wave amplifier 204, whichpreferably also takes the form of an automatic gain controlled amplifierproviding an adjustable sensing threshold as a function of the measuredR-wave amplitude. A signal is generated on P-out line 206 whenever thesignal sensed between electrodes 317 and 321 exceeds the present sensingthreshold. The general operation of the R-wave and P-wave amplifiers 200and 204 may correspond to that disclosed in U.S. Pat. No. 5,117,824, byKeimel, et al., issued Jun. 2, 1992, for an Apparatus for MonitoringElectrical Physiologic Signals, incorporated herein by reference in itsentirety. However, any of the numerous prior art sense amplifiersemployed in implantable cardiac pacemakers, defibrillators and monitorsmay also usefully be employed in conjunction with the present invention.

Switch matrix 208 is used to select which of the available electrodesare coupled to wide band amplifier 210 for use in digital signalanalysis. Selection of electrodes is controlled by the microprocessor224 via data/address bus 218, which selections may be varied as desired.Signals from the electrodes selected for coupling to bandpass amplifier210 are provided to multiplexer 220, and thereafter converted tomulti-bit digital signals by A/D converter 222, for storage in randomaccess memory 226 under control of direct memory access circuit 228.Microprocessor 224 may employ digital signal analysis techniques tocharacterize the digitized signals stored in random access memory 226 torecognize and classify the patient's heart rhythm employing any of thenumerous signal processing methodologies known to the art.

Telemetry circuit 330 receives downlink telemetry from and sends uplinktelemetry to the patient activator by means of antenna 332. Data to beuplinked to the activator and control signals for the telemetry circuitare provided by microprocessor 224 via address/data bus 218. Receivedtelemetry is provided to microprocessor 224 via multiplexer 220. Theatrial and ventricular sense amp circuits 200, 204 produce atrial andventricular EGM signals which also may be digitized and uplinktelemetered to an associated programmer on receipt of a suitableinterrogation command. The device may also be capable of generatingso-called marker codes indicative of different cardiac events that itdetects. A pacemaker with marker-channel capability is described, forexample, in U.S. Pat. No. 4,374,382 to Markowitz, incorporated byreference herein in its entirety. The particular telemetry systememployed is not critical to practicing the invention, and any of thenumerous types of telemetry systems known for use in implantable devicesmay be used. In particular, the telemetry systems as disclosed in U.S.Pat. No. 5,292,343 issued to Blanchette et al., U.S. Pat. No. 5,314,450,issued to Thompson, U.S. Pat. No. 5,354,319, issued to Wyborny et al.U.S. Pat. No. 5,383,909, issued to Keimel, U.S. Pat. No. 5,168,871,issued to Grevious, U.S. Pat. No. 5,107,833 issued to Barsness or U.S.Pat. No. 5,324,315, issued to Grevious, all incorporated herein byreference in their entireties, are suitable for use in conjunction withthe present invention. However, the telemetry systems disclosed in thevarious other patents cited herein which are directed to programmableimplanted devices, or similar systems may also be substituted. Thetelemetry circuit 330 is of course also employed for communication toand from an external programmer, as is conventional in implantableanti-arrhythmia devices.

The device of FIG. 2 includes an optional activity sensor 344, mountedto the interior surface of the device housing or to the hybrid circuitwithin the device housing and corresponds to sensor 30 of FIG. 1. Thesensor 344 and sensor present in circuitry 342 may be employed in theconventional fashion described in U.S. Pat. No. 4,428,378 issued toAnderson et al, incorporated herein by reference in its entirety, toregulate the underlying pacing rate of the device in rate responsivepacing modes.

The remainder of the circuitry is dedicated to the provision of cardiacpacing, cardioversion and defibrillation therapies, and, for purposes ofthe present invention may correspond to circuitry known in the priorart. An exemplary apparatus is disclosed for accomplishing pacing,cardioversion and defibrillation functions as follows. The pacertiming/control circuitry 212 includes programmable digital counterswhich control the basic time intervals associated with DDD, VVI, DVI,VDD, AAI, DDI, DDDR, VVIR, DVIR, VDDR, AAIR, DDIR and other modes ofsingle and dual chamber pacing well known to the art. Circuitry 212 alsocontrols escape intervals associated with anti-tachyarrhythmia pacing inboth the atrium and the ventricle, employing, any anti-tachyarrhythmiapacing therapies known to the art.

Intervals defined by pacing circuitry 212 include atrial and ventricularpacing escape intervals, the refractory periods during which sensedP-waves and R-waves are ineffective to restart timing of the escapeintervals and the pulse widths of the pacing pulses. The durations ofthese intervals are determined by microprocessor 224, in response tostored data in memory 226 and are communicated to the pacing circuitry212 via address/data bus 218. Pacer circuitry 212 also determines theamplitude of the cardiac pacing pulses under control of microprocessor224.

During pacing, the escape interval counters within pacer timing/controlcircuitry 212 are reset upon sensing of R-waves and P-waves as indicatedby signals on lines 202 and 206, and in accordance with the selectedmode of pacing on time-out trigger generation of pacing pulses by paceroutput circuits 214 and 216, which are coupled to electrodes 317, 321,324 and 326. The escape interval counters are also reset on generationof pacing pulses, and thereby control the basic timing of cardiac pacingfunctions, including anti-tachyarrhythmia pacing.

The durations of the intervals defined by the escape interval timers aredetermined by microprocessor 224, via data/address bus 218. The value ofthe count present in the escape interval counters when reset by sensedR-waves and P-waves may be used to measure the durations of R-Rintervals, P-P intervals, PR intervals and R-P intervals, whichmeasurements are stored in memory 226 and are used in conjunction withthe present invention to determine oversensing and in conjunction withtachyarrhythmia detection functions.

Microprocessor 224 operates as an interrupt driven device, and isresponsive to interrupts from pacer timing/control circuitry 212corresponding to the occurrences of sensed P-waves and R-waves andcorresponding to the generation of cardiac pacing pulses. Theseinterrupts are provided via data/address bus 218. Any necessarymathematical calculations to be performed by microprocessor 224 and anyupdating of the values or intervals controlled by pacer timing/controlcircuitry 212 take place following such interrupts. Microprocessor 224includes associated ROM in which the stored program controlling itsoperation as described below resides. A portion of the memory 226 may beconfigured as a plurality of recirculating buffers, capable of holdingseries of measured intervals, which may be analyzed in response to theoccurrence of a pace or sense interrupt to determine whether thepatient's heart is presently exhibiting atrial or ventriculartachyarrhythmia.

The arrhythmia detection method of the present invention may include anyof the numerous available prior art tachyarrhythmia detectionalgorithms. One preferred embodiment may employ all or a subset of therule-based detection methods described in U.S. Pat. No. 5,545,186 issuedto Olson et al. or in U.S. Pat. No. 5,755,736 issued to Gillberg et al.,both incorporated herein by reference in their entireties. However, anyof the various arrhythmia detection methodologies known to the art mightalso usefully be employed in alternative embodiments of the invention.

In the event that an atrial or ventricular tachyarrhythmia is detected,and an anti-tachyarrhythmia pacing regimen is desired, timing intervalsfor controlling generation of anti-tachyarrhythmia pacing therapies areloaded from microprocessor 224 into the pacer timing and controlcircuitry 212, to control the operation of the escape interval counterstherein and to define refractory periods during which detection ofR-waves and P-waves is ineffective to restart the escape intervalcounters.

In the event that generation of a cardioversion or defibrillation pulseis required, microprocessor 224 employs the escape interval counter tocontrol timing of such cardioversion and defibrillation pulses, as wellas associated refractory periods. In response to the detection of atrialor ventricular fibrillation or tachyarrhythmia requiring a cardioversionpulse, microprocessor 224 activates cardioversion/defibrillation controlcircuitry 230, which initiates charging of the high voltage capacitors246, 248 via charging circuit 236, under control of high voltagecharging control line 240. The voltage on the high voltage capacitors ismonitored via VCAP line 244, which is passed through multiplexer 220 andin response to reaching a predetermined value set by microprocessor 224,results in generation of a logic signal on Cap Full (CF) line 254,terminating charging. Thereafter, timing of the delivery of thedefibrillation or cardioversion pulse is controlled by pacertiming/control circuitry 212. Following delivery of the fibrillation ortachycardia therapy the microprocessor then returns the device tocardiac pacing and awaits the next successive interrupt due to pacing orthe occurrence of a sensed atrial or ventricular depolarization. In theillustrated device, delivery of the cardioversion or defibrillationpulses is accomplished by output circuit 234, under control of controlcircuitry 230 via control bus 238. Output circuit 234 determines whethera monophasic or biphasic pulse is delivered, whether the housing 311serves as cathode or anode and which electrodes are involved in deliveryof the pulse.

FIG. 3 is a portion of a stored electrogram showing near-field andfar-field pulses where there is an indication of a false positivenear-field pulse. As illustrated in FIG. 3, the near-field signal 40 isrecorded between the tip and ring electrodes of the bipolar sensinglead, such as electrodes 24 and 26, for example. This signal is input toa sense amplifier that senses voltages that exceed a threshold. Thefar-field signal 42 is recorded between secondary electrodes such as thelead coil and the can or a sensing lead in another part of the heart(left auricle or right ventricle). A marker channel 43 below far-fieldelectrogram 42 displays each sensed event from the near-field signal,such as Fibrillation Sense (FS), Fibrillation Detected (FD), TachycardiaSensed (TS) Ventricular Sense (VS) Capacitors charged (CE), or CapacitorDischarged (CD) for example. The numbers below the letters on markerchannel 43 indicate the time between sensed events. For example on theleft side of FIG. 3 there are two VS events, and the number below andbetween them is “670”, indicating that there were 670 millisecondsbetween the two VS events. Note that at the left of the electrogram wave40 is a relatively normal R-wave representation 44. The period ofrelative normal R-wave representation 44 is followed by a series oferratic signals 46 that indicate an oversensing problem (i.e., afractured lead conductor or insulation break on the lead).

An examination of far-field signal 42, however, shows a relativelyregular far-field R-wave. During the period of relative normal R-waverepresentation 44, the far-field signal 42 follows the near-field signal40 quite closely. When the near-field signal 40 becomes erratic in anerratic portion 46, the far-field signal 42 continues to show regularR-wave far-field pulses indicating that the erratic portion 46 may bedue to oversensing. As the near-field signal 40 recovers at a period ofrelative normal R-wave representation 48, the far-field signal 42continues to follow the near-field signal 40, suggesting that theirregular portion 46 of the near-field signal 40 was due to oversensing,and probably an intermittent failure, since the R-wave pulses ofnear-field signal 40 recovered at a period of relative normal R-waverepresentation 48.

With a pattern of this nature, it would be premature to deliver atherapy to the patient, particularly a painful defibrillation shock, inresponse to erratic portion 46 sensed in far-field signal 40. Typicallyseveral methods are used to avoid delivering a shock under theseconditions. First, if there is a detection of an irregularity as seen inthe erratic portion 46 of near-field signal 40, one can wait to seewhether the problem goes away by increasing the number of intervals fordetection (as is the case in the waves of FIG. 3), which would suggestthat the problem may be an oversensing problem and not an arrhythmia.Also, the sensing lead electrode configuration could be changed, andpacemakers may be programmed to automatically change the sensing leadconfiguration (e.g. bipolar to unipolar). Finally, the patient could begiven an alert (a vibration or audio alert, for example) to advise thepatient to see his doctor to have the ICD and its leads checked, but analert would not prevent the shock at the moment of oversensing.

FIG. 4 is a portion of an electrogram showing near-field and far-fieldR-wave sensing pulses where there is an actual cardiac episode requiringtherapy. The near-field signal 40 and the far-field signal 42 are shownas in FIG. 3. In this case the beginning (left side) of near-fieldsignal 40 shows relatively normal R-waves in portion 50, although thepulses are inverted from those of FIG. 3. Likewise far-field signal 42confirms the regularity during portion 50. At portion 52 of thenear-field signal 40, however, a highly irregular waveform exists.Unlike in FIG. 3, however, the far-field wave 42 does not maintain aregular R-wave periodicity during portion 50, but rather confirms theirregularity of near-field signal 40. This would strongly suggest anarrhythmia in the patient's ventricle and call for therapy in the formof a defibrillation shock. As above, however, certain intermediate stepsmay be taken before actually administering the shock such as waiting ashort period of time (perhaps ten or fifteen seconds) to see whether thesituation resolves itself. This period of time occurs because thecapacitors are charging. If in fact this waveform identifies anarrhythmia event, a therapy must be administered very quickly.

The decision to administer a therapy has been based primarily upon thenear-field R-wave. The present invention uses the far-field electrogramto discriminate QRS complexes between supraventricular (e.g. sinustachycardia, atrial fibrillation) and ventricular arrhythmias. In thisway, the present invention provides an algorithm that takes into accountother information to provide a better determination of an actualarrhythmia before subjecting a patient to a painful defibrillationshock.

FIG. 5 is a flow chart of a method of delivering a therapy in animplantable medical device, according to an embodiment of the presentinvention. As illustrated in FIG. 5, each time a V-sense signal issensed between a near-field sensor, i.e., electrodes 24 and 26corresponding to a next beat, Block 500, a determination is made as towhether the sensed event is a VF event, with a counter corresponding tothe number of sensed events and number of VF events being incremented inorder to generate a number of intervals for detection of ventricularfibrillation (VF NID), Block 502. In either case, i.e., the event is nota VF event or the event is determined to be a VF event, a determinationis made as to whether a predetermined number of VF events M have beendetected, Block 504, by determining whether the number of intervals fordetection of ventricular fibrillation is greater than the predeterminednumber M. If the predetermined number of VF events M has not beendetected, NO in Block 504, the process waits for the next beat to occur,Block 500. Once the predetermined number of VF events M have beendetected, YES in Block 504, a baseline measure associated with afar-field signal associated with the beat that is sensed betweensecondary electrodes is determined, Blocks 506-512, as described below.The secondary electrodes for sensing the far-field signal can includethe lead coil 20 and the uninsulated portion of the housing 11, forexample, or a sensing lead 6, 15 in another part of the heart alone orin combination with the uninsulated portion of the housing 11. Inaddition, the far-field sensing electrodes could also include one of thenear-field electrodes.

According to one embodiment of the present invention, for example, thepredetermined number M is set as three events so that once three VFevents are detected, a baseline measure is determined, Blocks 506-512,for each subsequently sensed beat.

FIGS. 6A and 6B are graphical representations of a determination of abaseline measure of a far-field signal according to an embodiment of thepresent invention. In particular, as illustrated in FIGS. 5, 6A and 6B,in order to determine the baseline measure for the sensed beatsoccurring after the predetermined number of VF events M have beendetected, voltage values 609 associated with a predetermined number ofsamples located within a predetermined window 600 of a far-field signal602 that is centered around the sensed beat 604 are determined, Block506. According to an embodiment of the present invention, window 600 isset as a 188 ms window, for example, so that voltage values aredetermined for 12 samples using a 64 Hz sampling rate.

Both a variance, such as a standard deviation SD, for example, of thevoltage values of the 12 samples, Block 508, and a central tendency,such as a mean, a median, or a sum, for example, of the absolute valuesof the voltage values of the twelve samples is determined, and so thatthe baseline measure is calculated using the product of the variance andthe central tendency of the absolute values, Block 512. In addition,according to an embodiment of the present invention, the baselinemeasure is determined by being set equal to one of either the varianceor the central tendency, rather than the product of both.

In addition to determining a baseline measure associated with thecurrent sensed beat, a determination is made as to whether the VF NID isgreater than a predetermined threshold, Block 514. For example,according to an embodiment of the present invention, a determination ismade in Block 514 as to whether 18 out of the last 24 beats weredetermined to be VF events. If the VF NID threshold is not reach, NO inBlock 514, the process waits for the next beat to be sensed, Block 500.Once the VF NID threshold has been met, YES in Block 514, adetermination is made as to whether the baseline measure determined forany of a predetermined number of previously sensed beats, such as thelast twelve beats, for example, or for a predetermined number of the 12beats, such as 2 or 3 of the 12 beats, for example, is less than apredetermined threshold, Block 516. If none of the baseline measuresassociated with the predetermined number of previously sensed beats isless than the predetermined threshold, NO in Block 516, no oversensingis determined to be occurring and VF detection is confirmed, Block 518.If any one of the baseline measures associated with the predeterminednumber of sensed beats is less than the threshold, YES in Block 516,oversensing is determined to likely be occurring, Block 520, such aswould result from a lead integrity failure, for example, and a patientalert, such as a vibration or audio alert, a wireless signal transmittedto a remote monitor, satellite, internet, for example, is activated toalert the patient, Block 522, and to advise the patient to see hisdoctor to have the ICD and its leads checked. Because the alert is notintended to prevent the shock at the moment of oversensing, the processis repeated for the next beat, Block 500, until a predetermined timeperiod associated with the charging of the capacitor(s) for deliveringthe shock, such as 10 seconds, for example, has expired, YES in Block524. Once the timer has expired, the VF detection process for deliveringa corresponding shock therapy continues according to the normal VFdetection process, Block 526, and therapy is delivered as determinednecessary.

According to an embodiment of the invention, once it is determined thatthe patient is experiencing a VF event, i.e., the VF NID is greater thanthe threshold (18 of last 24 beats are determined to be VF events) andoversensing is determined, the device begins charging of one or morecapacitors for delivering the therapy to the patient. According toanother embodiment of the invention, once it is determined thatoversensing is likely to be occurring, charging of the capacitors may bewithheld until the timer of Block 524 has expired, for example. In thisway, the algorithm of the present invention checks to see if thebaseline measure threshold has been satisfied for each beat at and afterVF detection, withholding charging and/or therapy until either thethreshold is not satisfied or the timer expires.

As illustrated in FIGS. 6A and 6B, in order to reduce the effects ofoversensing, the present invention evaluates the corresponding far-fieldsignal to determine whether a VF episode is also indicated in thefar-field signal, so that if the VF NID threshold is met in Block 516due to oversensing rather than the occurrence of a VF episode, such aswhen there is a loss in lead integrity resulting from lead fractures,corrupted connector interfaces, EMI issues, R-wave oversensing,myopotentials, etc., the patient is alerted of the possible oversensingissue. In particular, when VF is detected in the signal sensed by thenear-field sensor but not in the signal detected by the far-field sensor602, both the standard deviation of the voltage values of the samples inthe associated window 600 and the absolute values of the voltage valueswill be negligible since the far-field signal will likely approach theisoelectric baseline value of the far-field EGM signal, as illustratedin FIG. 6A. However, when VF is detected both in the signal sensed bythe near-field sensor and in the signal detected by the far-field sensor602, both the standard deviation of the voltages of the samples in theassociated window 600 and the absolute values of the voltages will begreater. Therefore the product of the standard deviation and the sum ofthe absolute values, Block 512, will be small when there is oversensingcompared to when an actual arrhythmia event is occurring. In particular,according to an embodiment of the present invention, the threshold ofBlock 516 is set equal to one, so that if the product of the standarddeviation and the sum of the absolute values is less than one for any ofthe predetermined number of previously sensed events, it is likely thatthe isoelectric baseline of the far-field signal is occurring andtherefore oversensing is likely occurring.

FIG. 7 is a flow chart of a method of delivering a therapy in a medicaldevice, according to an embodiment of the present invention. Asillustrated in FIG. 7, according to an embodiment of the presentinvention each time a V-sense signal corresponding to a next beat issensed between a near-field sensor, i.e., electrodes 24 and 26, Block700, a determination is made as to whether the sensed event is apredetermined event, such as a VF event, for example, with a countercorresponding to the number of sensed events and number of VF eventsbeing incremented in order to generate a number of intervals fordetection of the predetermined event (VF NID), Block 702. In eithercase, i.e., the event is not a VF event or the event is determined to bea VF event, a determination is made as to whether a predetermined numberof VF events M have been detected, Block 704, by determining whether thenumber of intervals for detection of ventricular fibrillation is greaterthan the predetermined number of VF events M. According to oneembodiment of the present invention, for example, the predeterminednumber of VF events M is set as three events.

If the predetermined number of VF events M has not been detected, NO inBlock 704, the process waits for the next beat to occur, Block 700. Asillustrated in FIGS. 7, 6A and 6B, once the predetermined number of VFevents M have been detected, YES in Block 704, voltage values associatedwith a predetermined number of samples located within a predeterminedwindow 600 located over a portion of the far-field signal 602 andcentered around the current sensed beat 604 are determined, Block 706.Window 600 is set as a 188 ms window, for example, so that voltagevalues are determined in window 600 for 12 samples using a 64 Hzsampling rate to generate 12 voltage values 609 associated with a sensedevent 608 within each window 600.

Once the voltage values 608 have been determined, a sample amplitude isgenerated for the current sensed event 608, Block 708, by determining amaximum voltage value and a minimum voltage value of the 12 voltagevalues 609 for the far-field window 600 associated with the sensed event608, and determining the difference between the maximum voltage valueand the minimum voltage value. The process is repeated for the nextsensed beats, generating a sample amplitude for each of the subsequentlysensed events. Once the sample amplitude is generated for the sensedbeat, a determination is made as to whether an episode requiringtherapy, such as ventricular fibrillation for example, is detected bydetermining whether the VF NID is greater than a predeterminedthreshold, Block 710. For example, according to an embodiment of thepresent invention, an episode requiring therapy is determined to bepresent in Block 710 when 18 out of the last 24 beats are determined tobe VF events. If the VF NID threshold is not reached, NO in Block 710,the process waits for the next beat to be sensed, Block 700 and isrepeated for the next sensed beat.

Once an episode requiring therapy is detected, YES in Block 710, amaximum sample amplitude and a minimum sample amplitude associated witha predetermined number of the sensed events 606, such as 8 sensedevents, for example, is determined, Block 712. Although thepredetermined number of sensed events 606 illustrated in FIG. 6A isshown as including eight sensed events, it is understood that themaximum sample amplitude and a minimum sample amplitude of any desirednumber of sensed events could be utilized and the invention is notintended to be limited to the use of eight sensed events. Adetermination is then made as to whether the maximum sample amplitude isgreater than a predetermined maximum sample amplitude threshold, such as2 mv for example, and the minimum sample amplitude is less than apredetermined minimum sample amplitude threshold, such as 1 mv forexample, Block 714.

If the maximum sample amplitude is not greater than the predeterminedmaximum sample amplitude threshold or the minimum sample amplitude isnot less than the predetermined minimum sample amplitude threshold, NOin Block 714, the process waits for the next beat to be sensed, Block700, and is then repeated for the next sensed event. If the maximumsample amplitude is greater than the predetermined maximum sampleamplitude threshold and the minimum sample amplitude is less than thepredetermined minimum sample amplitude threshold, YES in Block 714, adetermination is made as to whether the minimum sample amplitude is lessthan a predetermined percentage of the maximum sample amplitude, Block716. For example, according to an embodiment of the present invention,the determination in Block 716 involves determining whether the minimumsample amplitude is less than one sixth of the maximum sample amplitude,although any desired percentage may be utilized. It is understood thatalthough 2 mv and 1 mv are utilized as the maximum and minimum sampleamplitude thresholds, respectively, any desired value may be utilizedfor the predetermined maximum and minimum sample amplitude thresholds.

If the minimum sample amplitude is greater than or equal to apredetermined percentage of the maximum sample amplitude, NO in Block716, no oversensing is determined to be occurring and VF detection isconfirmed, Block 718. If the minimum sample amplitude is less than apredetermined percentage of the maximum sample amplitude, YES in Block716, oversensing is determined to likely be occurring, Block 720, suchas would result from a lead integrity failure, for example, and apatient alert, such as a vibration or audio alert, a wireless signaltransmitted to a remote monitor, satellite, internet, for example, isactivated to alert the patient, Block 722, and to advise the patient tosee his doctor to have the ICD and its leads checked. Because the alertis not intended to prevent the shock at the moment of oversensing, theprocess is repeated for the next beat, Block 700, until a predeterminedtime period associated with the charging of the capacitor(s) fordelivering the shock, such as 10 seconds, for example, has expired, YESin Block 724. Once the timer has expired, the VF detection process fordelivering a corresponding shock therapy continues according to thenormal VF detection process, Block 726, and therapy is delivered asdetermined necessary.

As described above, according to an embodiment of the invention, once itis determined that the patient is experiencing a VF event, i.e., the VFNID is greater than the threshold (18 of last 24 beats are determined tobe VF events) and oversensing is determined, the device begins chargingof one or more capacitors for delivering the therapy to the patient.According to another embodiment of the invention, once it is determinedthat oversensing is likely to be occurring, charging of the capacitorsmay be withheld until the timer of Block 724 has expired, for example.

FIG. 8 is a graphic representation of maximum and minimum amplitudes ofsensed events for delivery of therapy by a medical device according tothe present invention. In the graphical representation of FIG. 8, bothactual VT/VF events 802 from a VT/VF episode and non-VT/VF events 804that are most likely the result of oversensing caused by instances oflead failure, for example, sensed by the far-field electrodeconfiguration are plotted on a graph of maximum amplitudes (X) versusminimum amplitudes (Y). As can be seen in FIG. 8, the non-VT/VF events804 tend to occur within an area 806 defined by boundaries correspondingto the maximum sample amplitude being greater that 2 mv 808, the minimumsample amplitude being less that 1 mv 810, and the minimum sampleamplitude being less than one sixth of the maximum sample amplitude 812,as described above in the embodiment of FIG. 7.

It is understood that while the determination of the sample amplitude inBlock 708 is described as being generated by determining the differencebetween a maximum and a minimum voltage value of the 12 voltage values609, the present invention may utilize other methods of determining thesample amplitudes and therefore is not intended to be limited to thedetermination of a maximum and a minimum amplitude as described. Forexample, according to an embodiment of the present invention, the sampleamplitude may be determined in Block 708 using the secondlargest/smallest voltage value, or the third largest/smallest voltagevalue, etc., or may include looking at a range between values and soforth. In addition, according to an embodiment of the present invention,the determination of whether oversensing is occurring could be performedusing only the threshold determination in Block 716. In particular, thedetermination of Block 714 could be eliminated so that once an episoderequiring therapy is detected, YES in Block 710, the minimum and themaximum sample amplitudes associated with the predetermined number ofsensed events 606 are determined in block 712, and then thedetermination is made as to whether the minimum sample amplitude is lessthan the predetermined percentage of the maximum sample amplitude, Block716.

Some of the techniques described above may be embodied as acomputer-readable medium comprising instructions for a programmableprocessor such as microprocessor 224. The programmable processor mayinclude one or more individual processors, which may act independentlyor in concert. A “computer-readable medium” includes but is not limitedto any type of computer memory such as floppy disks, conventional harddisks, CR-ROMS, Flash ROMS, nonvolatile ROMS, RAM and a magnetic oroptical storage medium. The medium may include instructions for causinga processor to perform any of the features described above fordelivering therapy in an implantable medical device according to thepresent invention.

While at least one exemplary embodiment has been presented in theforegoing detailed description, it should be appreciated that a vastnumber of variations exist. It should also be appreciated that theexemplary embodiment or exemplary embodiments are only examples, and arenot intended to limit the scope, applicability, or configuration of theinvention in any way. Rather, the foregoing detailed description willprovide those skilled in the art with a convenient road map forimplementing the exemplary embodiment or exemplary embodiments. Itshould be understood that various changes can be made in the functionand arrangement of elements without departing from the scope of theinvention as set forth in the appended claims and the legal equivalentsthereof.

1. A method of controlling delivery of therapy in a medical device,comprising: sensing cardiac signals and detecting cardiac events via afirst electrode configuration; determining the presence of an episodeevent in response to the detected cardiac events; sensing the cardiacsignals via a second electrode configuration; determining predeterminedvalues associated with cardiac signals sensed via the second electrodeconfiguration corresponding to the event episode; and controllingdelivery of therapy in response to the determined values, whereindetermining predetermined values comprises: determining voltage valuesof a predetermined number of samples located within a window definedabout the cardiac signals sensed via the second electrode configuration;determining a sample maximum voltage value and a sample minimum voltagevalue corresponding to the predetermined number of samples; andcomparing the sample maximum voltage value and the sample minimumvoltage value to generate sample amplitudes.
 2. The method of claim 1,wherein the comparing comprises determining the difference between thesample maximum voltage value and the sample minimum voltage value. 3.The method of claim 1, wherein controlling delivery of therapycomprises: determining a maximum amplitude and a minimum amplitudecorresponding to a predetermined number of the generated sampleamplitudes; determining whether the maximum amplitude is greater than amaximum amplitude threshold and the minimum amplitude is less than afirst minimum amplitude threshold; and determining whether the minimumamplitude is less than a second minimum amplitude threshold.
 4. Themethod of claim 3, wherein the predetermined number of samples is 12samples, and the predetermined number of the generated sample amplitudesis 8 sample amplitudes.
 5. The method of claim 3, wherein the maximumamplitude threshold is equal to 2 mv and the first minimum amplitudethreshold is equal to 1 mv.
 6. The method of claim 3, wherein the secondminimum amplitude threshold corresponds to a percentage of the maximumamplitude.
 7. The method of claim 3, further comprising generating apatient alert in response to the minimum amplitude being less than thesecond minimum amplitude threshold.
 8. The method of claim 3, furthercomprising suspending delivery of the therapy up to a predetermined timeperiod in response to the minimum amplitude being less than the secondminimum amplitude threshold.
 9. The method of claim 1, whereincontrolling delivery of therapy comprises: determining a maximumamplitude and a minimum amplitude corresponding to a predeterminednumber of the generated sample amplitudes; and comparing the maximumamplitude and the minimum amplitude.
 10. The method of claim 9, whereinthe comparing comprises determining whether the minimum amplitude isless than a percentage of the maximum value.
 11. A medical device,comprising: means for sensing cardiac signals and detecting cardiacevents via a first electrode configuration; means for determining thepresence of an episode event in response to the detected cardiac events;means for sensing the cardiac signals via a second electrodeconfiguration; means for determining predetermined values associatedwith cardiac signals sensed via the second electrode configurationcorresponding to the event episode; and means for controlling deliveryof therapy in response to the determined values, wherein the means fordetermining predetermined values comprises: means for determiningvoltage values of a predetermined number of samples located within awindow defined about the cardiac signals sensed via the second electrodeconfiguration; means for determining a sample maximum voltage value anda sample minimum voltage value corresponding to the predetermined numberof samples; and means for comparing the sample maximum voltage value andthe sample minimum voltage value to generate sample amplitudes.
 12. Thedevice of claim 11, wherein the means for comparing determines thedifference between the sample maximum voltage value and the sampleminimum voltage value.
 13. The device of claim 11, wherein the means forcontrolling delivery of therapy comprises: means for determining amaximum amplitude and a minimum amplitude corresponding to apredetermined number of the generated sample amplitudes; means fordetermining whether the maximum amplitude is greater than a maximumamplitude threshold and the minimum amplitude is less than a firstminimum amplitude threshold; and means for determining whether theminimum amplitude is less than a second minimum amplitude threshold. 14.The device of claim 13, wherein the predetermined number of samples is12 samples, and the predetermined number of the generated sampleamplitudes is 8 sample amplitudes.
 15. The device of claim 13, whereinthe maximum amplitude threshold is equal to 2 mv and the first minimumamplitude threshold is equal to 1 mv.
 16. The device of claim 13,wherein the second minimum amplitude threshold corresponds to apercentage of the maximum amplitude.
 17. The device of claim 13, furthercomprising means for generating a patient alert in response to theminimum amplitude being less than the second minimum amplitudethreshold.
 18. The device of claim 13, wherein the means for controllingdelivery of therapy suspends therapy up to a predetermined time periodin response to the minimum amplitude being less than the second minimumamplitude threshold.
 19. The method of claim 11, wherein the means forcontrolling delivery of therapy comprises: means for determining amaximum amplitude and a minimum amplitude corresponding to apredetermined number of the generated sample amplitudes; and means fordetermining whether the minimum amplitude is less than a predeterminedpercentage of the maximum amplitude.
 20. A computer readable mediumhaving computer executable instructions for performing a methodcomprising: sensing cardiac signals and detecting cardiac events via afirst electrode configuration; determining the presence of an episodeevent in response to the detected cardiac events; sensing the cardiacsignals via a second electrode configuration; determining amplitudesassociated with a predetermined number of cardiac signals sensed via thesecond electrode configuration corresponding to the event episode; andcontrolling delivery of therapy in response to the determinedamplitudes, wherein determining amplitudes comprises: determiningamplitudes of a predetermined number of samples located within a windowdefined about the cardiac signals sensed via the second electrodeconfiguration; determining a sample maximum amplitude and a sampleminimum amplitude corresponding to the predetermined number of samples;and comparing the sample maximum amplitude and the sample minimumamplitude to generate sample amplitudes.
 21. The computer readablemedium of claim 20, wherein controlling delivery of therapy comprises:determining a maximum amplitude and a minimum amplitude corresponding toa predetermined number of the generated sample amplitudes; determiningwhether the maximum amplitude is greater than a maximum amplitudethreshold and the minimum amplitude is less than a first minimumamplitude threshold; and determining whether the minimum amplitude isless than a second minimum amplitude threshold.
 22. The computerreadable medium of claim 20, wherein controlling delivery of therapycomprises: determining a maximum amplitude and a minimum amplitudecorresponding to a predetermined number of the generated sampleamplitudes; and determining whether the minimum amplitude is less than apredetermined percentage of the maximum amplitude.